The present invention relates to a fuel cell system, which supplies the fuel gas containing hydrogen and the oxidant gas containing oxygen to generate electric power with the chemical reaction between the hydrogen and oxygen, and more particularly relates to a fuel cell system having a heating device for the humidified fuel and oxidant gases.
Various types of electric vehicles have been developed recently, on which a traction motor is mounted instead of the conventional engine. As one example of these types of electric vehicles, fuel cell vehicles have been developed rapidly, on which a hydrogen ion exchange membrane fuel cell (hereinafter referred to as PEM fuel cell) that is shortly referred to as PEMFC (Proton Exchange Membrane Fuel Cell) is mounted as the power source for the traction motor.
The PEM fuel cell has a stack structure, in which a large number of cells, its of power generation, are stacked. Each cell interposes a membrane-electrode joint called MEA (membrane Electrode Assembly) between the anode separator with hydrogen supplying path and the cathode separator with oxygen supplying path. The MEA has the anode electrode catalyst layers and the gas diffusion layers layered alternately on one side, and the cathode electrode catalyst layers and the gas diffusion layers layered alternately on the other side of the hydrogen ion exchange membrane of solid polymer material called PEM (Proton Exchange Membrane).
In this type of PEM fuel cell, the humidified hydrogen gas as a fuel gas flows through the hydrogen supplying path from the anode inlet to outlet, and the humidified air as an oxidant gas flows through the oxygen supplying path from the cathode inlet to outlet. Then the hydrogen ions travel from the anode to cathode of each cell, permeating the PEM (ion exchange membrane) of the moistened MEA to produce the electric power of about 1 volt. In this case the PEM fuel cell is capable of producing electric power most stably under the temperature of 70 to 80 degrees Celsius.
The fuel cell system with the PEM fuel cell having the mechanism of power generation described above has the following means to control the temperature of the fuel cell as well as to continue power generation by the continuous supply of the humidified air and hydrogen gas: an air supply line for cooling the air by the intercooler, which is forcefully fed by the supercharger, and supplying it to the cathode inlet; an air discharge line for discharging the moisture rich residual air from the cathode outlet; a hydrogen gas supply line for supplying the stored hydrogen gas to the anode inlet: a hydrogen gas discharge line for discharging the moisture rich residual hydrogen gas from the anode outlet; one humidifying apparatus of water permeable membrane for humidifying the air of the air supply line by means of the moisture exchange with the moisture rich residual air of the air discharge line; another humidifying apparatus of water permeable membrane for humidifying the hydrogen gas of the hydrogen gas supply line by means of the moisture exchange with the moisture rich residual hydrogen gas of the hydrogen gas discharge line; and a cooling line for controlling the temperature of the fuel cell by circulating the cooling fluid between the fuel cell and the heat exchanger.
In this connection, a humidifier of light and compact hollow fiber membrane (see Japanese Laid-Open Patent 7-71795) is generally used for the humidifying apparatus of water permeable membrane described above. As the means of humidifying the hydrogen gas of the hydrogen gas supply line, an ejector is also employed to suck the moisture to the hydrogen gas supply line, which flows into the hydrogen gas discharge line. On the other hand, as the cooling line described above, a two-stage cooling is generally used to prevent the liquid junction of the fuel cell, in which the aqueous solution of ethylene glycol series, electrically non-conductive, is circulated as first cooling fluid between the fuel cell and the first heat exchanger of fluid-fluid type, and the second cooling fluid is circulated between the first heat exchanger and the second heat exchanger (radiator) of gas-liquid type.
The humidifier of hollow fiber membrane described above has a hollow fiber membrane module, which contains a large number of the water permeable hollow fiber membranes in a bundle in a cylinder-like housing, and head blocks connected to the respective ends of the module. In this type of hollow fiber membrane humidifier, the dry air of the air supply line flows as a sweep gas from one to another head block through the cylinder-like housing. Simultaneously, the moisture rich residual air of the air discharge line flows inside the respective hollow fiber membranes of a bundle as a cathode off-gas in the opposite direction. The moisture rich residual air of the air discharge line is dehumidified and the dry air of the air supply line is humidified by the moisture exchange between the former passing inside the respective hollow fiber membranes and the latter passing over the outer circumferential surfaces of the respective hollow fiber membranes. The porous hollow fiber membrane, which is permeable to the moisture in the gas by capillary condensation, is generally used for the hollow fiber membrane described before, since it has a good feature of high beat resistance. The non-porous hollow fiber membrane (ex. NAFION (RTM) of Du Pont) is also used, which is permeable only to the moisture in the gas by ion hydration.
The performance of power generation of the fuel cell system will diminish, when the temperature of the fuel cell is low at starting and the temperature of the air supplied to the cathode inlet by the air supply line is low. Especially when the ambient temperature is low, a problem of remarkable decrease in the power generation of the fuel cell will occur, since the temperature of the air supplied to the cathode inlet falls further and the humidification decreases owing to the fall of dew-point of the humidified air. Another problem that the durability of the fuel cell is degraded will occur. On the other hand, when the hydrogen gas of the hydrogen gas supply line is humidified by the ejector for water suction, the same problems described above will occur in the case of the fuel cell starting or the low ambient temperature, since the temperature of the hydrogen gas supplied to the anode inlet falls due to the latent heat of vaporization resulting from the ejector operation.
The object of the present invention is to provide a fuel cell system, which is capable of preventing the decrease in the power generation of the fuel cell and further the degradation of durability of the fuel cell as well, even if the temperature of the fuel cell is low at starting or the ambient temperature is low.
The present invention to address the above issues provides a fuel cell system, which comprises; a cooling line for cooling a fuel cell with the cooling fluid circulating between the fuel cell and a heat exchanger; one humidifying apparatus of water permeable membrane type, which humidifies the oxidant gas supplied to the cathode inlet by means of a moisture exchange with the moisture rich cathode off-gas discharged from the cathode outlet of the fuel cell; another humidifying apparatus of water permeable membrane type, which humidifies the fuel gas supplied to the anode inlet by means of a moisture exchange with the moisture rich off-gas discharged from the outlet of the anode or cathode of the fuel cell; a heating device for heating the oxidant and fuel gases with the cooling fluid of the cooling line, which absorbs the heat from the fuel cell and flows into the heat exchanger.
In the fuel cell system according to the present invention, when the oxidant gas is supplied to the cathode inlet and the fuel gas is supplied to the anode inlet on its starting, it starts generating electric power and discharging the moisture rich residual cathode off-gas from the cathode outlet and the moisture rich anode off-gas from the anode outlet. This moisture rich cathode off-gas and the oxidant gas supplied to the cathode inlet exchange the moisture in the humidifying apparatus of water permeable membrane thus the discharged cathode off-gas dehumidified and the supplied oxidant gas humidified. Similarly, the moisture rich off-gas discharged from the outlet of the anode or cathode and the fuel gas supplied to the anode inlet exchange the moisture in the humidifying apparatus of water permeable membrane, thus the discharged off-gas dehumidified and the supplied fuel gas humidified. The cooling fluid of the cooling line circulates between the fuel cell and the heat exchanger to control the temperature of the fuel cell. In so doing, the beating device heats the humidified oxidant gas supplied to the cathode inlet and the humidified fuel gas supplied to the anode gas respectively, utilizing the cooling fluid, which has absorbed the heat from the fuel cell and flows into the heat exchanger.
If the heating device is so arranged that it may heat the respective oxidant and fuel gases by heating their humidifying apparatus, it will be preferable to increase the humidification of the oxidant and fuel gases, since these humidifying apparatus can humidify the oxidant and fuel gases with the high dew-point temperatures.
The heating device may be arranged so that the oxidant gas is heated by one heat exchanger provided in the oxidant gas supplying path and the fuel gas by another heat exchanger in the fuel gas supplying path. In this case, if the respective heat exchangers are placed in the upstream of the respective humidifiers, it will be preferable to increase the humidification of the oxidant and fuel gases rapidly, since the dew-point temperatures can be raised by heating the oxidant and fuel gases supplied to the humidifying apparatus in advance.
The heating device may also be arranged so that it heats the oxidant gas through the intercooler provided in the upstream of the humidifying apparatus in the supplying path of the oxidant gas. This arrangement allows the rise of its dew-point temperature by raising the temperature of oxidant gas supplied to the humidifying apparatus in advance, thereby increasing the humidification of the oxidant gas by the humidifying apparatus rapidly. Also it obviates additional heat exchangers, enabling a compact fuel cell system. It would be preferable to prepare a switching device such as a three-way valve for the circulating path of the cooling fluid of the cooling line, which is capable of switching between the cooling fluid flowing into the heat exchanger after absorbing the heat from the fuel cell and the cooling fluid flowing into the fuel cell after radiating in the heat exchanger.
Further, the heating device may be arranged so that it heats the fuel gas by heating the ejector for water suction provided in the supplying path of the fuel gas. In this case, it would be preferable to increase the humidification of the fuel gas by the ejector for water suction, since the ejector for water suction prevents the temperature drop of the fuel gas caused by the latent heat of vaporization while it humidifies the fuel gas.